A mechanical seal device seals fluids by a pair of sliding elements sliding in close contact through their sliding surfaces. Therefore, as the sliding elements of this mechanical seal device, a material with a low friction and superior wear resistance is selected. In particular, sliding elements sliding in a non-lubricated state with no fluid lubrication film supplied between the sliding surfaces are required to have superior self-lubricating ability and wear resistance. For sliding elements sliding in the non-lubricated state, there has been used, for example, a carbon material or a polytetrafluoroethylene resin material containing various materials (for example, graphite, glass fiber, carbon fiber, metal powder, etc.) as an aggregate.
On the other hand, when using a carbon material as the material forming the above sliding elements, the carbon ends up losing its lubricating ability in an atmosphere such as a vacuum, a reducing gas or dry air with a low dew point, so that it is sometimes remarkably worn.
Further, even when using a polytetrafluoroethylene resin material containing an aggregate as the material forming the above sliding elements, the effect of the above atmospheres is relatively low compared with a carbon material, but still wear ends up occurring.
In particular, in stirring machines etc. used in the process of production of food or pharmaceuticals, from the viewpoint of the influence on sanitation or the properties of the product, it is necessary to prevent the entry of impurities. Therefore, in a mechanical seal device used in this stirring machine etc., to prevent the entry of wearing powder (impurities) into the product as much as possible, it is required to reduce wear of the sliding elements as much as possible even in the non-lubricated state.
For this reason, to reduce the wearing of the sliding elements, in sliding elements comprised of polytetrafluoroethylene resin including this aggregate, the types and ratios of the aggregates contained have been studied to improve the wear resistance (for example, Patent Article 1).
However, in a polytetrafluoroethylene resin sliding material containing an aggregate, to maintain the lubricating ability, it is difficult to make the content of the polytetrafluoroethylene resin having low wear resistance less than 70%, so that there were limits to improvement of the wear resistance. Therefore, it was impossible to use this for sliding elements for the various equipments used in the process of production of food and pharmaceuticals.
On the other hand, in recent years, a ceramic or metal material containing a self-lubricating material such as carbon has been sometimes used for non-lubricated sliding elements, but when conditions such as a vacuum state and pressurized state are repeated, the sliding surfaces get galling or ringing due to poor lubrication etc. occurs well as generating wearing powders. Therefore, it has been attempted to provide grooves in the sliding surfaces so as to form a film of gas between the sliding surfaces at the time of sliding and thereby lightening the load of the sliding elements (for example, Patent Article 2). However, even when providing the sliding surfaces with grooves in this way, a sufficient effect could not be obtained under low rotational speed conditions or conditions where the above-mentioned vacuum state or pressurized state are repeated.    [Patent Article 1] Japanese Unexamined Patent Publication (A) No. 2001-26792    [Patent Article 2] Japanese Unexamined Patent Publication (A) No. 5-60247